Unequal IVO & IVC events

[Dairymilkbatman]

Your reported numbers show IVO to be 7° late vs. spec and IVC 5° early, so only a 2° difference. Thus 'by the numbers', the cam is now only 1° retarded, so I'd say congratulations, you are done*! . (*Can be debated, but 1° is close enough for me.)

It would only be a check of the cam machining, but for peace of mind you could back off the checking lift point (say 0.008"/0.20 mm for a start), to bring the measured duration closer to spec., thus allowing direct confirmation. Ditto for the ICL.

! ! My first cam! I'm very happy.

With 0.20 checking I got:
8° BTDC IVO
51° ABDC IVC

Having trouble returning to 0(0.20mm). +/- 0.5. Checked x3 and got these numbers twice

[Dairymilkbatman]

I got
IVO @ 4° ATDC
and
IVC @ 38° ABDC
So with a duration of 222 I am getting there, right?

Are we concentrating on the duration because of my incompetence with measuring and reading a cam card? lol
You guys were right, I wasn't including the lash.
I am so grateful for the help. In the UK, there is little to no engine building - at least not like the US. Finding information here or someone who know's is hard.

So where do I go from here? once I reach 226 of course.

If your numbers above are correct, your duration at the measured lift is 38+180-4 = 214.

More importantly, your lobe center is 4 + (214/2) = 111 ATDC, which is close to the spec 110. Race engines tend to use more like 105/105 at least in Fiat twincams for more overlap. Street is often 110/110 like yours.

Exhaust cam center, BTDC is calculated as follows: ((EVO+180+EVC)/2) - EVC. This is assuming that EVC is a ATDC value, not BTDC. If BTDC, then use a negative number for the EVC.

Do the same for exhaust to check lobe center as shown above.

Below is my cam timing chart with 107 centers. You could put your lift points on top of my chart and compare. The intake is a 294 deg advertised, the exhaust is 300deg but has a lot shorter actual duration because lobe profile is less aggressive and more valve clearance.

Ok I'll take a look. The only thing I don't understand from the above is when to use addition or subtraction? Is the math different between the Intake and Exhaust?

As for the wheel, I've order a new 11" from summit. I put another pointer/wire to almost touching the outside diameter of the wheel and did a rotation. It began scraping shortly ABDC then a gap grew moving back to TDC. Guess I could put a feeler gauge between the pointer and wheel

[modok]

176+38=214
not 222
Perhaps you added 4 instead of subtracting it.
Getting closer tho!

Don't sweat it, this IS tricky. I can make it sound simple but,
that's just because I"m naturally good at it, AND lots of practice

Oh I see, am I right in saying I would use subtraction if the IVO ATDC? and addition if the event is BTDC?

ah...yes

[Nut124]

It helps to draw the timing circle, like the one attached.

In cam timing math, the origin of the numbers is assumed to be as follows:

IVO: Deg BTDC
IVC: Deg ABDC
ICL: Deg ATDC

EXO: Deg BBDC
EXC: Deg ATDC
ECL: Deg BTDC

Duration is VO +180 + VC. ICL is Duration/2 - IVO. ECL is duration/2 - EVC.

In your case, because the amount of lift used for the measurement, the IVO was ATDC. If you plot your values on the circle, you will get it. Good luck.

If that is a Stg II cam, then my intake cam looks rather large. Is yours a fuel injected engine?

[Dairymilkbatman]

Think I'm getting the hang of this

Could someone check my numbers and math?

Initial checking @ 1.25mm:
39° BBDC EVO
8° BTDC

I'm right in thinking I need to reduce the checking lift?
Then @ 1.20mm:
38° BBDC EVO
8° BTDC
Thought I'd try 0.35mm:
49° BBDC ECO
1° ATDC EVC

Retarded cam by 2° and got:
46 BBDC EVO
5° ATDC EVC
Dur: 231
ECL: 110.5

Spec:
Duration: 270
Peak Diff @ max lift: 110 BTDC
With checking 0.35mm 65° BBDC EVO, 25 ATDC EVO
With checking 1.25mm 45° BBDC EVO, 5° ATDC EVC


Thanks so much for the help, you guys are awesome.
I went back and read the entire thread again knowing what I've learned and a lot more made sense. I missed a lot of what you guys were trying to portray, thank you for the patience!

[Nut124]

Your EX centerline math seems correct: 110.5.

Does your engine have variable valve timing? If not, I would consider erring in the other direction for more overlap, power: 109/109.

If it has variable timing, the VVT will likely change the CLs to something like 105/105 at high rpm.

[MadBill]

Right on; great work Dairy!
Now to lock in the knowledge for future reference, you need to walk someone else through the process...
If you want to get an idea of how important the last degree of precision is, check the open/close points on one or more other cylinders.